Our goals are to understand the mechanisms of (1) assembly of myosin-II into bipolar filaments, (2) cytokinesis and (3) organelle movements by myosin-I. To reach these goals, we propose the following studies: 1. Assembly and function of myosin-II. We will determine the atomic structure of the C-terminal part of the tail of Acanthamoeba myosin-II that is required for the formation of minifilaments. We will compare the assembly kinetics of C-terminal fragments with intact myosin tails to test our hypothesis that assembly is favored by intramolecular diffusion. 2. Regulation of cytokinesis. We will use biochemistry, genetics and molecular genetics to study cytokinesis in Schizosaccharomyces pombe. We will determine genetic interactions between the two isoforms of myosin-II and gene products known to participate in cytokinesis. After identifying and characterizing the light chains associated with the two isoforms of myosin-II, we will use genetic screens to identify new components that regulate the time and position of furrow formation. 3. Interaction of myosin-I with membranes. We will characterize lipid binding properties of Acanthamoeba myosin-I isozymes, analyze the assembly of myosin-I tails into higher order structures in solution and on lipid surfaces and determine which parts of the myosin-I molecule are required to target the three isoforms to specific membranes in vivo. To aid in this analysis, we will attempt to crystallize the tail myosin- I for high resolution structure determination. This work will contribute to our understanding of basic cellular process such as cell division (as important feature of cancer), cellular motility (important for tumor metastasis), phagocytosis (important in defense against microbial pathogens) and muscle contraction.